1. Field of the Invention
This invention in general relates to the field of injection molding and in particular to self-contained hot runner systems.
2. Description of the Prior Art
In plastic injection molding processes, a thermoplastic or thermoset molding compound is first heated to plasticity in an injection cylinder at controlled temperature. Afterwards, the plasticized compound is forced from the cylinder through a nozzle by means of pressure generated within the cylinder. After emerging from the nozzle, the plasticized compound passes through a hole in a mold plate, usually stationary, and is then conveyed along a flow channel(s) to the mold cavity. The flow channel, depending on mold architecture, may comprise one or more sprue bushings and/or runner systems, which may or may not be heated for temperature control. Once in the cavity, the molten resin assumes the shape of the cavity and then is cooled to the point where it solidifies, acquiring the external shape of the cavity. The mold is then opened, and the part ejected or otherwise removed. The entire process is usually automated with clamping and unclamping of mold parts prior to injection and after part ejection, removal after cooling, taking place under the control of a microprocessor or other form of automated controller.
For best process performance regardless of part size, it is known to be beneficial to control the temperature of the plasticized compound more or less constant throughout its travel to the cavity. This reduces process problems associated with material degradation due to thermal variability, improves yield by decreasing scrap losses, and increases machine production time by reducing down time due to freeze offs.
However, while standardized in many respects, particularly with respect to mold base or plate thicknesses, present molding machinery does not always provide for precise temperature control to take advantage of its beneficial effects. Indeed, much of the available machinery is still run employing poor control techniques allowing the temperature of the resin to vary from the time it leaves the nozzle until it reaches a zone in the mold where temperature control is reacquired with, for example, internal heating channels in the mold.
Early on in mold practice, the most commonly used injection mold was one with a conventional runner system fed by a sprue. Such designs incorporated traditional unheated or cold sprue bushings to accept the melt delivered from the molding machine nozzle. Standard sprue bushings employed were available in a variety of styles and lengths to meet many anticipated molding applications. Such bushings, as those of more recent design, while unheated were provided with standard spherical radii and orifice sizes to be compatible with available nozzles configurations.
Here, the runner system was cut at the parting line to route plastic to the cavities. Full round runners were, and still are, the most efficient and popular because they afforded the least heat loss per unit volume of material flow. While offering an economical approach to mold construction, the conventional runner system was most frequently employed for short-run applications. Its major disadvantage is the requirement for degating parts and the need for regrinding of the runners and sprue, both labor intensive operations which do not readily lend themselves to automation.
Three plate molds were an improvement over the conventional mold approach in that they can automatically degate parts in the molding cycle and also allow the part to be gated on the top, usually a more desirable position for round parts. Because of the three plate scheme, two additional parting lines are available to allow automatic separation of the runner from the part. However, the three plate approach by itself afforded no additional advantage in terms of thermal control.
Consequently, no matter what type of molding strategy is employed, the runner and or runner and sprue system must still function to get the material to the cavity with a minimum of loss of temperature and pressure. To achieve this purpose, those in molding arts have employed a number of approaches.
One is the insulated runner mold. Very few of these are built today because other runnerless molding technologies perform much better than this type. They are interesting, however, for historical reasons. This mold design relied on a very thick runner system whose outside regions would solidify with heat loss to form insulation at the outside of the runner. Closer to the center higher temperature was maintained because of lower heat loss due to the outside insulating properties thus allowing the new melt to flow through the center in an open flow path. While easy to build, this style of mold was extremely difficult to run, particularly when cycle interruptions occurred. If new material was not frequently introduced into the system, the insulated runner would freeze, and the cull would have to be physically removed from the mold. As this was a frequent occurrence, the runner plates were latched together, and the machine clamping pressure was relied on to keep the plates from separating under injection pressure. While successful under some circumstances, like fast cycles and large shots using particular plastics, this type of mold largely is a thing of the past.
Next in terms of least expense for initial cost and continued maintenance is the internally heated runnerless molding system. In this system, material from the machine nozzle enters through a heated nozzle locator into the flow channel where heat is supplied to the plastic by a thermocouple cartridge heater located inside a distributor tube held in position by a end caps. Melt is distributed to probes or to secondary distributor channels through either round bores in solid plates or trapezoidal channels in split-plate designs. The molten plastic flows along the gate probe to the gate and into the cavity.
These type of systems are relatively insusceptible to material leakage. They are constructed of solid blocks with gun-drilled distributor channels. The distributor plates are retained with allen head cap screws of sufficient strength to withstand the molding injection pressures. Such systems are usually comprised of pre-engineered components with application information being supplied on varied mini-prints for the moldmaker to design an build a mold compatible with available styles. As plastic is heated from the inside out, less power is required than with externally heated systems. Probes, located near the gate, provide heat to provide thermal control at the gate, allowing drool-free molding without gate freeze-up. Cycle interruptions of up to five minutes are possible without freezing the gate in these type of systems.
Externally heated runnerless molding systems called manifolds are heated, with either round cartridges or cast-in heaters, to a temperature sufficient to keep the plastic being processed in a molten condition. The machine nozzle mates to a nozzle seat in the form of a replaceable contact area. Plastic flows from the molding machine, through the nozzle seat, to a lead-in channel, and then into a primary flow channel. The primary channel delivers the plastic to bushing drops. The flow channels are bored into the manifold to form channels for plastic to flow. Additional flow channels may connect, forming secondary or even tertiary flow paths before ending at bushing drop locations. Specific bushing diameters and flow channel sizes are recommended based on flow volumes and material viscosity. Thermocouples are located at several locations within the manifold to monitor temperature and provide feedback information for control purposes.
More sophisticated practice, apparently not yet universally accepted, recognizes the advantages of controlling temperature by employing hot sprue bushings to convey material from the nozzle to the cavity gate often times through the fixed mold plate, sometimes referred to as the "A" plate or base. A variety of approaches for providing heat in these hot sprue bushings have been used. Among these are the use of resistive heating elements and heat pipes such as those described in U.S. Pat. No. 4,034,952 entitled "HOT PLASTIC INJECTION BUSHING" issued on Jul. 12, 1977. In the latter case, the heat pipes are used to transfer heat from electrically powered heater bands located at the nozzle end of the sprue bushing to regions along the bore near the tip.
In multicavity applications capable of providing more than one part per molding cycle, hot sprue bushings and internally heated molds, including those employing manifolds, generally occupy a large percentage of mold real estate per cavity. Consequently, it is a primary object of this invention to provide a highly reliable self-contained hot runner system that will provide the art with flexibility in the design, manufacture and operation of multicavity molds.
It is another object of this invention to provide a multi-tip hot runner system that may have its tips easily replaced or changed for a different style.
It is yet another object of the present invention to provide a self-contained hot runner system with only one thermocouple and yet provide balanced temperature and pressure control throughout its delivery architecture.
It is yet another object of the present invention to provide a self-contained multitip hot runner system available in a variety of configurations employing varying numbers of tips.
It is yet another object of the present invention to provide a self-contained hot runner system having an interface for use with an injection machine nozzle or a manifold.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter. A full understanding of the invention will best be had from reading the detailed description to follow in connection with the detailed drawings.